Implantable drug infusion devices are used to provide patients with a constant or programmable long term dosage or infusion of a drug or any other therapeutic agent. Essentially such device may be categorized as either active or passive.
Active drug or programmable infusion devices feature a pump or a metering system to deliver the drug into the patient's system. An example of such an active drug infusion device currently available is the Medtronic SynchroMed.TM. programmable pump. Such pumps typically include a drug reservoir, a peristaltic pump to pump out the drug from the reservoir, and a catheter port to transport the pumped out drug from the reservoir via the pump to a patient's anatomy. Such devices also typically include a battery to power the pump as well as an electronic module to control the flow rate of the pump. The Medtronic SynchroMed.TM. pump further includes an antenna to permit the remote programming of the pump. Needless to say, in view of these various components, the cost as well as the size of active drug infusion devices is greater than desired.
Passive drug infusion devices, in contrast, do not feature a pump, but rather rely upon a pressurized drug reservoir to deliver the drug. Thus such devices tend to be both smaller as well as cheaper as compared to active devices. An example of such a device includes the Medtronic IsoMed.TM.. This device delivers the drug into the patient through the force provided by a pressurized reservoir. In particular, this reservoir is pressurized with a drug to between 20 to 40 psi (1.3 to 2.5 bar) and is used to deliver the drug into the patient's system. Typically the flow path of the drug from the reservoir to the patient includes a flow restrictor, which permits a constant flow rate. The flow rate, however, is only constant, if the pressure difference between reservoir and patient does not change. Factors that could impact this pressure difference include temperature, pressure-volume dependence of reservoir and altitude, among others. The selected pressure for the reservoir is thus typically quite high, so that absolute pressure changes only cause small and acceptable errors in flow rate. This also requires, however, the drug to be injected into the reservoir using still higher pressure. This is often a very difficult to achieve using a hand operated syringe.
In addition such devices present challenges to accurately deliver a precise dosage of drug to the patient. As the amount of drug is removed from the reservoir, the pressure in the reservoir drops. This, in turn, affects the flow rate such that only over a limited pressure range will the flow rate be constant. Still further, because the ambient pressure changes in which the patient exists (due to weather or altitude for example) the resistance to drug infusion likewise changes, further affecting the flow rate. Temperature will also have a similar impact.
Thus there is a need for a drug infusion system which will permit the drug flow rate to be independent of reservoir pressure within a given pressure range.